[[Test-Driven Development (TDD)]] and the [[You Aren't Gonna Need It (YAGNI)]] principle naturally complement each other by focusing on what is essential and valuable at each stage of the [[Software Development Life Cycle (SDLC)]]. In TDD, you begin by writing a test before the actual code. This test represents the client's needs or requirements—essentially, test code is client code. By thinking from the client's perspective, whether that client is a person, subsystem, or external system, you ensure that the code meets business needs without unnecessary complexity. This approach is akin to [[User-centered Design]], as it prioritizes solving the client’s problems with the simplest, most direct solution. Since the test drives the design, it prevents the implementation of unnecessary features that aren’t required to pass the test. TDD’s focus on writing the minimum code necessary to pass a test naturally aligns with YAGNI. You start with a failing test that reflects an immediate requirement, then write just enough code to make that test pass. This process reveals the essential concepts and structures needed to solve the problem without being sidetracked by potential future requirements that may never be needed. As you continue with TDD, you write additional tests for new cases or features. Each new test helps to further clarify the essential nature of the model you’re building. This iterative process of refining the design based on real, tested requirements helps you solidify the core concepts necessary for the system’s functionality. It also helps prevent over-engineering by keeping your focus on the current needs reflected by the tests. Because TDD discourages unnecessary features (each feature must be justified by a test), the resulting codebase is leaner and more maintainable. This aligns with YAGNI by reducing system complexity and making it easier to understand, maintain, and extend. By implementing only what’s necessary, the codebase remains adaptable to change. As new requirements emerge, you can add new tests and extend the model in a controlled, deliberate manner, ensuring that the code evolves in response to real needs rather than speculative ones. As I often say, [[You pay for what’s there, and what’s not can’t break]]—a fancy way of saying that [[Code is liability]].